The present invention relates to an arrangement for converting thermal energy to mechanical energy in a vehicle.
When fuel is burnt in a combustion engine, chemical energy is converted to mechanical energy. A substantial part of the chemical energy is not converted to mechanical energy but to thermal energy which is released to the surroundings in various ways. An example of this is the thermal energy in the exhaust gases which are discharged to surrounding air. A known practice is to use WHR (waste heat recovery) systems in vehicles to capture thermal energy and convert it to mechanical energy. A WHR system comprises a pump which circulates a working medium in a line circuit. The line circuit comprises an evaporator in which the working medium is warmed so that it is vaporised and extra-heated by means of a heat source which may with advantage be exhaust gases from a combustion engine, and a turbine which is driven by the extra-heated gaseous working medium. When the working medium expands through the turbine, part of its thermal energy is converted to mechanical energy which may be used for immediate operation of the vehicle or be converted to and stored as electrical energy.
A known practice where a heavy vehicle is travelling on a long downhill run is to use an automatic braking process which gives the vehicle a constant speed downhill by activation of one or more supplementary brakes. The supplementary brakes may be retarders, exhaust brakes or compression brakes. To activate the automatic braking process, the vehicle is given a desired speed, followed by the driver operating the vehicle's brake pedal, a button or a suitable lever to initiate activation of the braking process. An electrical control unit controls the braking process so that the vehicle maintains the desired speed all the way down the hill. Using supplementary brakes for this braking process saves unnecessary wear on the vehicle's wheel brakes and eliminates the risk of their overheating. The braking process ends when the driver activates the vehicle's accelerator pedal or clutch pedal. The thermal energy generated when a supplementary brake, e.g. a hydraulic retarder, is activated is cooled away by the engine's cooling system. The engine's cooling system will be under heavy load when a hydraulic retarder is activated, and long hills may entail risk of overheating the coolant in the cooling system.